ClangParser: fix builtin includes

Commit 14cb69d added the "/bpftrace/include" prefix to headers
generated by bpftrace (__btf_generated_header.h, clang_workarounds.h),
however, commit bc73244 ("Don't unpack kernel headers or look in tmp
(#3156)") broke the latter when moving code around in main.cpp.

Reintroduce the prefix.

src/main.cpp

@@ -414,7 +414,7 @@ static void parse_env(BPFtrace& bpftrace)
       }
     }
     extra_flags.push_back("-include");
-    extra_flags.push_back(CLANG_WORKAROUNDS_H);
+    extra_flags.push_back("/bpftrace/include/" CLANG_WORKAROUNDS_H);
 
     for (auto dir : include_dirs) {
       extra_flags.push_back("-I");